Acute pain and chronic pain differ in their etiology, pathophysiology, diagnosis and treatment. Acute pain is self-limiting and serves a protective biological function by acting as a warning of on-going tissue damage. It is a symptom of a disease process experienced in or around the injured or diseased tissue. Acute pain is nociceptive in nature, and occurs secondary to chemical, mechanical and thermal stimulation of A-delta and C-polymodal pain receptors.
Chronic pain, on the other hand, serves no protective biological function. Rather than being the symptom of a disease process, chronic pain is itself a disease process. Chronic pain is unrelenting and not self-limiting and as stated earlier, can persist for years and even decades after the initial injury. Chronic, non-malignant pain is predominately neuropathic in nature and involves damage either to the peripheral or central nervous systems. Following a peripheral nerve injury (eg. crush, stretch, or axotomy) sensitization occurs which is characterized by spontaneous activity by the neuron, a lowered threshold for activation and increased response to a given stimulus.
Neuropathic pain, which is pain initiated or caused by a primary lesion or dysfunction in the nervous system are estimated to have a prevalence in USA of 1.5%, meaning in all 4 million people is affected by the conditions [1].
It manifests itself as a chronic state of hyperexcitability in the part of the nervous system transmitting the pain. Special pain receptors exist the so called nociceptors collected in the transient receptor potential receptor superclass Trp. They include the temperature sensitive vanilloid receptors (TrpV), the acid sensitising ion channels and mechano receptors [2].
Far from all lesion or diseases in the nervous system give rise to neuropathic pain. This variation is due to a genetic predisposition [3]. Thus about 8% of conventional operations where nerves inevitable is cut results in chronic pain conditions related to the scar region [4].
Pain is classified in nociceptive-, inflammatory- and neuropathic pain [5]. The first caused by a noxious stimulus is of short duration, the inflammatory pain is longer lasting and is provoked by the inflammatory mediators of the disease process which lower the threshold for pain perception. The threshold normalises when the inflammation is healed. In case of neuropathic pain, the lowered threshold induced by the injury in the affected region does not return to normal, it is so to speak irreversible set to a lower threshold than normal. This means that a stimulus not causing pain in a healthy person will give rise to pain sensation in the affected area in a patient. If a stimuli below what a normal person may sense at all is felt painful, it is called allodynia. Hyperalgesia is used to characterize the increased intensity of the pain sensation that a neuropathic person experience when a “normal” painful stimulus is applied [6].
No causal cure exists today for the neuropathic pain conditions. Only insufficient pain relieve can be offered by the healthcare system [1,7]. For the classical analgesics like opioids and non-steroidal anti-inflammatory drugs the dose-response curve is shifted to the right, meaning that much larger dose is needed for pain relief [8,9]. Anaesthetics of Na+ channel blockers like carbamazepine, lamotrigine and lidocain show effect when given systematically but they produce severe adverse effects [10,11]. Treatment with anticonvulsants like gabapentin which works by decreasing the Ca2+ influx in neurons has proven effective to dampen the pain intensity, but not to neutralise it completely. Tricyclic antidepressants mediate their effect through a range of transmitter systems e.g. the serotonergic and noradrenergic ones. They are acting both centrally and in the periphery. They are effective but the adverse effect profile is not attractive [12].
Thus there is an urgent need for drugs that modulate/interfere specifically with the molecular processes that regulate the threshold level in the affected nerve tissue to avoid the initiation or development of the processes leading to the lower threshold level for pain. Another mechanism could be constantly to suppress the molecular events that keep the threshold at the lower mode.
Analgesia
If better drugs have to be developed it is necessary to address targets that are central to the pain generation through initiation and regulation of the pain threshold level [3].
The sensitization of the pain threshold level that takes place through inflammation and involve a long row of mediators—the so called the inflammatory soup, which among others include serotonin, histamine, prostaglandins, bradykinin, substance P, calcitonin gene related peptide (CGRP), nerve growth factor (NGF) and various cytokines. In relation to nerve injury the neurotrophin NGF may be particularly important although other mediators may also participate. NGF is a member of the neurotrophin family that comprises NGF, BDNF, NT-3 and NT-4. The neurotrophins can interact with the ‘non-selective’ receptor p75NTR, as well as their cognate tyrosine receptor kinase; NGF binds preferentially to tyrosine receptor kinase A (TrkA); BDNF and NT4 to TrkB; and neurotrophin 3 (NT3) to TrkC. These interactions have generally been considered to be of high affinity. However, in reality, the binding of NGF to TrkA, and of BDNF to TrkB, is of low affinity, but it can be regulated by receptor dimerization, structural modifications or association with the p75NTR. The p75 receptor can bind to each neurotrophin, and also acts as a co-receptor for Trk receptors, sortilin and other Vps10p-domain receptors (cf. FIG. 1).
Once NGF binds, a TrkA two stages event can take place: First, at the surface of sensory neurons, TrkA physically interacts with TrpV1, a member of a large family of calcium-permeable nonselective cation channels, that is implicated in development of thermal hyperalgesia induced by tissue injury and inflammation, and in chronic and neuropathic pain [13]. Binding of NGF to TrkA strongly potentiates calcium-influx by TrpV1 and the induction of pain. Second, NGF binding to TrkA and p75NTR also results in retrograde transport of the complex back to the cell body of the neurons where it modulates the activity and transcription of proteins important for neuronal survival and differentiation, but also of enzymes and ion-channels involved in the nociceptive transmission [3,14]. Also microglia may play a role in this activation [15].
After injury of a nerve, the amount of NGF decreases in the sensory neurons, but after a while it increases again due to secretion from Schwann cells, macrophages and likely also by the neurons themselves. This stimulates the regeneration of the nerve.
Further it has been shown that proNGF, a precursor form of NGF, interacts with a heteromer receptor complex consisting of p75NTR and Sortilin thus forming a ternary complex. As opposed to the trophic activities elicited by mature NGF, proNGF induces apoptosis by the formation of said ternary complex via p75NTR and Sortilin, unless proNGF is converted into its mature form [24].
Earlier [16] related severe pain sensory neuropathies to the presence of the TrkA gene in mice. Arrett et al. (2007) [17] examined the expression of the proNGF, sortilin and p75NTR in the dorsal root ganglion. They found that a subpopulation of neurons coexpressed sortilin and p75NTR and that nerve injury induced a severe loss of a subpopulation of the small p75NTR-sortilin coexpressing neurons.
Thus, these observations points to the relation between the development and maintenance of chronic and neuropathic pain conditions.
In the example section of this patent application, further experimental evidence is demonstrated for how an agent can be designed to specifically interact with the signalling cascade involved in the development of the neuropathic pain conditions described herein above, through interruption of the formation of the receptor complex.
The Vps10p-domain Receptor Family
The present inventors have studied the effect of modulation formation of a pain signalling complex, wherein said complex comprises a Vps10p-domain receptor:TrpV receptor binary complex or a Vps10p-domain receptor:TrpV receptor ternary complex, such as a complex between a Vps10p-domain receptor, TrkA, and a TrpV receptor.
The members of the Vps10p-domain receptors are Sortilin, SorLA, SorCS1, SorCS2 and SorCS3.
Sortilin
Sortilin, the archetypal member of the Vps10p-domain receptor family is occasionally also referred to as Neurotensin receptor 3 (NTR3), Glycoprotein 95 (Gp95) or 100 kDa NT receptor. Human Sortilin is accessed in Swiss Prot under ID No. Q99523.
Sortilin, (SEQ ID NO. 1) is a type I membrane receptor expressed in a number of tissues, including the brain, spinal cord, testis, liver and skeletal muscle [18-19]. Sortilin belongs to a family of receptors comprising Sortilin, SorLA [20], SorCS1, SorCS2 and SorCS3.
All the receptors in this family share the structural feature of an approximately 600-amino acid N-terminal domain with a strong resemblance to each of the two domains which constitute the luminal portion of the yeast sorting receptor Vps10p [21]. The Vps10p-domain (Vps10p-D) that among other ligands binds neurotrophic factors and neuropeptides [22-26], constitutes the entire luminal part of Sortilin (sSortilin) and is activated for ligand binding by enzymatic propeptide cleavage. Sortilin is a multifunctional type-1 receptor capable of endocytosis as well as intracellular sorting [22, 23], and as shown recently, it also engages in signaling by triggering proneurotrophin-induction of p75NTR-mediated neuronal apoptosis [24, 25, 30, 31]. Sortilin is synthesized as a proprotein, which is converted to mature Sortilin by enzymatic cleavage and removal of a short N-terminal propeptide. Only the mature receptor binds ligands and interestingly, all its known ligands, e.g. Neurotensin (NT), lipoprotein lipase, the proforms of nerve growth factor-β (proNGF) and brain derived neurotrophic factor (proBDNF), receptor associated protein (RAP), and its own propeptide, compete for binding [23-25, 28], indicating that the diverse ligands target a shared or partially shared binding site. NT is a tridecapeptide, which binds to Sortilin, SorLA and the two G-protein coupled receptors NTR1 and NTR2 [22, 32-34].
SorLA
Sorting protein-related receptor abbreviated SorLA (Swiss prot ID no Q92673), also known as LR11, is a 250-kDa type-1 membrane protein and the second member identified in the Vps10p-domain receptor family SorLA, like sortilin, whose lumenal domain consists of a Vps10p domain only, is synthesized as a proreceptor that is cleaved by furin in late Golgi compartments. It has been demonstrated [33] that the truncation conditions the Vps10p domain for propeptide inhibitable binding of neuropeptides and the receptor-associated protein. In transfected cells, about 10% of full-length SorLA is expressed on the cell surface capable mediating endocytosis. The major pool of receptors is found in late Golgi compartments, and interaction with newly synthesized ligands has been suggested.
SorCS1-3
SorCS1 (Swiss prot ID no Q8WY21), SorCS2 (Swiss prot ID no Q96PQ0) and SorCS3 (Swiss prot ID no Q9UPU3) constitute a subgroup of mutually highly similar proteins containing both a Vps10p-D and a leucine-rich domain bordering the transmembrane domain [26, 38].
TrkA
Neurotrophic tyrosine kinase, receptor, type 1, also known as NTRK1 (Swiss prot ID no P04629) and normally referred to as TrkA (Tropomyosin-Related Kinase A) is a member of the neurotrophic tyrosine kinase receptor (NTKR) family. TrkA is the high affinity catalytic receptor for the neurotrophin, Nerve Growth Factor, or “NGF”. As such, it mediates the multiple effects of NGF, which includes neuronal differentiation and survival: upon NGF binding, TrkA dimerize and autophosphorylates which results in activation of downstream members of the MAPK pathway.
The presence of TrkA leads to cell differentiation and may play a role in specifying sensory neuron subtypes. Hence mutations in this gene have also been associated with congenital insensitivity to pain [39,40,41].
TrkA is known to associate with TrpV1 and this interaction is essential for NGF-mediated potentiation of TrpV1 activity [42].
Known agents with antagonistic properties include the kinase domain inhibitor K252A.
TRPV1
The Transient receptor potential vanilloid 1, TrpV1 (Swiss prot ID no Q8NER1), also known as the vanniloid receptor or the capsaicin receptor, is a nonselective ligand-gated cation channel that may be activated by a wide variety of exogenous and endogenous stimuli, including heat greater than 43° C., low pH, anandamide, N-arachidonoyl-dopamine, and capsaicin. TRPV1 receptors are highly expressed in a subset of dorsal root ganglia neurons, show a much higher sensitivity to heat than most ion channels and are involved in the transmission and modulation of pain, as well as the integration of diverse painful stimuli [43,44].
TRPV1 antagonist agents include AMG517 (highly selective was dropped out of clinical trials due to the undesirable side effects), SB-705498 and capsazepine.